Engines can be coupled to emission control devices, such as catalytic converters, to reduce exhaust emissions. However, these devices can become contaminated with sulfates, for example. In order to remove these contaminates, the temperature of the emission control device is raised significantly and a near stoichiometric air-fuel ratio is provided that alternates, or oscillates, around stoichiometry (between lean and rich).
One type of engine exhaust system routes all of the engine cylinders into a single exhaust path. One approach for raising temperature of such a single exhaust path sequentially operates some cylinders lean, and then some rich to create heat. Such an approach is described in U.S. Pat. No. 5,974,788, for example.
The inventors herein have recognized a disadvantage with such an approach. In particular, the transition from operating the cylinders lean to operating rich can cause a torque disturbance since all of the cylinders have to make this air-fuel ratio transition. While it is possible to utilize ignition timing adjustments to reduce this disturbance, such control is difficult to manage when transitioning from lean to rich values since a large variation in engine air-fuel ratio occurs, making it difficult to determine how much change in ignition timing is required.
Another approach to raising exhaust temperature is to operate some cylinders lean and other rich, so that rich and lean exhaust gasses react in the exhaust manifold, or emission control device. Such a system is described in U.S. Pat. No. 6,189,316.
Here, there is no transition required. However, the inventors herein have recognized other disadvantages with this approach. In particular, while operating some cylinder lean and others rich generates heat via reaction across surface precious metals of oxidants (from lean gasses) and reactants (reductants from rich gasses), it does not take advantage of oxidant storage effects to generate heat. As such, the heat is generated predominately in the first upstream catalyst that mixes the two gas streams. However, this may not be where heat is desired. Also, this approach requires significant ignition timing retard on the rich bank to compensate for the large torque difference.
The above disadvantages are overcome by: a system for an engine having at least a first group and a second group of cylinders, the system comprising:
an emission control device coupled at least to said first and second groups of cylinders; and
a computer storage medium having a computer program encoded therein for controlling fuel injected into the first and second group of cylinders, comprising:
code for, during a first interval, operating said first group of cylinders lean of stoichiometry and said second group of cylinders at stoichiometry; and
code for, during a second interval, operating said first group of cylinders at stoichiometry and said second group of cylinders rich of stoichiometry
In this way, it is possible to generate heat in a single bank exhaust system without requiring a transition of any cylinders from a highly lean air-fuel ratio to a highly rich air-fuel ratio. Note, such a transition could be used during some conditions, if desired; however, it is not required.
In this way, it is also possible to generate heat in a dual bank exhaust system, by creating at least two groups of cylinders on each bank of the engine.
In addition, operation in this way provides the advantage of minimizing the co-existence of rich and lean gases that generate exothermic heat by reacting across the surface of a precious metal in the catalyst. In other words, heat is generated primarily due to oxidant storage effects. In this way, it is possible to locate heat generation in a desired location in the exhaust system by placing oxidant storage in such desired locations. For example, if heat generation is primarily desired in a downstream emission control device, then by designing a system with greater oxidant storage downstream (rather than upstream), operation according to one aspect of the present invention can primarily provide downstream heat. Such advantages, as well as others, are described more fully below.
Note that various emission control devices can be used such as, for example, catalysts including platinum, palladium, and rhodium; or catalyst having platinum and barium, or various others.